Merge pull request dubesar#588 from sourabhvarshney111/Decorator

Added Decorator Pattern

Author: dubesar

Design Patterns/Decorator Pattern/DecoratorPattern.java

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+/*
+Reference: https://www.tutorialspoint.com/design_pattern/decorator_pattern.htm
+ */
+
+interface Shape {
+   void draw();
+}
+
+class Rectangle implements Shape {
+
+   @Override
+   public void draw() {
+      System.out.println("Shape: Rectangle");
+   }
+}
+
+class Circle implements Shape {
+
+   @Override
+   public void draw() {
+      System.out.println("Shape: Circle");
+   }
+}
+
+abstract class ShapeDecorator implements Shape {
+   protected Shape decoratedShape;
+
+   public ShapeDecorator(Shape decoratedShape){
+      this.decoratedShape = decoratedShape;
+   }
+
+   public void draw(){
+      decoratedShape.draw();
+   }    
+}
+
+class RedShapeDecorator extends ShapeDecorator {
+
+   public RedShapeDecorator(Shape decoratedShape) {
+      super(decoratedShape);        
+   }
+
+   @Override
+   public void draw() {
+      decoratedShape.draw();           
+      setRedBorder(decoratedShape);
+   }
+
+   private void setRedBorder(Shape decoratedShape){
+      System.out.println("Border Color: Red");
+   }
+}
+
+public class DecoratorPattern {
+   public static void main(String[] args) {
+
+      Shape circle = new Circle();
+
+      Shape redCircle = new RedShapeDecorator(new Circle());
+
+      Shape redRectangle = new RedShapeDecorator(new Rectangle());
+      System.out.println("Circle with normal border");
+      circle.draw();
+
+      System.out.println("\nCircle of red border");
+      redCircle.draw();
+
+      System.out.println("\nRectangle of red border");
+      redRectangle.draw();
+   }
+}
+

Design Patterns/Decorator Pattern/README.md

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+### Decorator Pattern
+
+### Introduction
+
+Decorator pattern allows a user to add new functionality to an existing object without altering its structure. This type of design pattern comes under structural pattern as this pattern acts as a wrapper to existing class.
+
+This pattern creates a decorator class which wraps the original class and provides additional functionality keeping class methods signature intact.
+
+### Advantages
+
+- It is flexible than inheritance because inheritance adds responsibility at compile time but decorator pattern adds at run time.
+- We can have any number of decorators and also in any order.
+- It extends functionality of object without affecting any other object.
+
+### Disadvantages
+
+- Code maintainability is difficult because this pattern creates lots of similar decorators which are sometimes hard to maintain and distinguish.
+
+### Programming
+
+- We are going to create a Shape interface and concrete classes implementing the Shape interface. We will then create an abstract decorator class ShapeDecorator implementing the Shape interface and having Shape object as its instance variable.
+
+- RedShapeDecorator is concrete class implementing ShapeDecorator.
+
+- DecoratorPatternDemo, our demo class will use RedShapeDecorator to decorate Shape objects.